1. Field of the Invention
This invention relates generally to a novel catalyst composition comprised of an aluminum halide and a metal alkoxide belonging to group IVB, for selectively oligomerizing olefins, present in a mixture of olefins, aromatics, paraffins and cycloparaffins, to polyolefins in good yields. A particular application of the invention is oligomerization of olefins contained in cracked refinery distillate streams to give polyolefins which, after the steps of distillation and stabilization by hydrogenation, give oligomers suitable for use as lubricating oil base stocks.
2. Description of the Related Art
Synthetic oil base stocks having viscosities of about 4 to 30 cSt or above at 100.degree. C. have been prepared in the prior art by oligomerization of olefins by conventional or modified Friedel-Craft catalysts. Thus, by contacting the alpha-olefins with boron trifluoride containing various promoters, synthetic oils suitable for lubricant use have been prepared, such as described in U.S. Pat. Nos. 4,400,565; 5,068,487 and 5,191,140. However, boron trifluoride is a pulmonary irritant and is fast being replaced by less hazardous catalysts like aluminum halides.
A number of aluminum halide catalyst systems have been disclosed for oligomerization of alpha-olefins to poly-alpha-olefins which could be used as lubricating oil base stocks possessing low pour points, higher viscosity index and good oxidation stability. U.S. Pat. No. 3,637,503 discloses the oligomerization of alpha-olefins having from 4 to 16 carbon atoms in the presence of aluminum chloride and a non-polymerizing hydrocarbon diluent. Similarly, aluminum chloride alone or along with organic promoters have been used to oligomerize alpha-olefins either pure alpha-olefins or alpha-olefins in the presence of a non-oligomerizing hydrocarbon diluent, see, for example, U.S. Pat. Nos. 5,196,635; 5,136,118; 4,107,080; 4,219,691 and 4,031,159. It is also known in the related field to oligomerize olefins in which the double bond is statistically distributed along the entire carbon chain. Thus, U.S. Pat. No. 4,167,534 discloses the oligomerization of olefins obtained from a PACOL-OLEX process, by contacting with aluminum chloride to obtain oligomers which, after distillation and catalytic hydrogenation, gave lubricating oil. Even though the feed stock for oligomerization was predominantly olefinic (up to 95%), the yield and the viscosity of the resulting oligomer were very poor.
However, all these processes pertain to oligomerization of either pure alpha-olefins or mixtures of pure alpha-olefins and, surprisingly, there are no reports on the utilization of linear olefins contained in refinery streams for the production of synthetic lubricant base stocks. Various refinery produced cracked distillate streams, particularly from Coker and FCC units, are quite rich in desired alpha-olefins which can be selectively concentrated by the process of urea adduction.
A major deficiency of the conventional Friedel-Craft catalysts is their inability to selectively oligomerize olefins in the presence of other unsaturated compounds like aromatics. However, aluminum chloride alone or with promoters is known to promote the alkylation of aromatics with olefins when applied to cracked refinery distillate streams which contained appreciable amounts of aromatics along with olefins. The oligomeric product thus obtained contain alkylated aromatics which made these products unsuitable for use as lubricating oils because of very poor oxidation stability. No prior art method either discloses or teaches any catalyst system which can selectively oligomerize olefins to polyolefins in the presence of aromatics. Consequently, it is in fact impossible to prepare an olefin oligomer having high viscosity index and high oxidation stability which could be qualified for such uses as gas turbine oil, hydraulic fluid for aircraft, crankcase oils, etc. by selective oligomerization of olefins present in the cracked refinery stream distillates which also contain aromatics, besides paraffins and cycloparaffins, by use of Friedel-Craft catalyst systems disclosed in the prior art. The present invention provides for a catalyst composition for preparation of olefin oligomers suitable for use as lubricating oils by selective oligomerization of olefins, contained in cracked refinery stream distillates, which have been processed through a step of urea adduction.
An object of the present invention is to propose a catalyst composition for selectively oligomerizing olefins present in cracked refinery stream distillates which are comprised of olefins, aromatics, paraffins and cycloparaffins having 8 to 20 carbon atoms.
A further object of this invention is to propose a catalyst composition which provides higher conversion of olefins present in the cracked refinery stream distillates, processed through a step of urea adduction and containing up to 5% aromatics.
Yet another object of the present invention is to propose a catalyst system for selective oligomerization of olefins contained in refinery distillate streams to produce oligomers having high viscosity index, low pour point and higher oxidation stability for use as base stocks in synthetic lubricants.